This disclosure relates to photopatterned growth of electronically active brush polymers for light emitting diode displays.
Display devices have become an integral part of society as a means of information transfer. In particular, organic light emitting diode (OLED) displays are among the most energy efficient 2D display technologies and can be found in everyday appliances, including smartphones, laptops, and televisions. Two aspects that have made efficient OLED displays possible are the use of phosphorescent materials and multi-colored pixel arrays. However, the energy efficiency of OLED displays is offset by the cost of production, in part, due to the use of evaporative deposition processes. Solution-based methods are attractive alternatives that grant access to low-cost large area and high throughput fabrication (e.g., spin-coating, ink-jet printing, roll-to-roll, and the like), but suffer from limited patterning capabilities. Thus, a simple method to generate patterned phosphorescent OLEDs from solution is particularly desirable.
The use of organic and organometallic phosphors have been critical useful to OLED device performance as a means to harness energy from triplets states, which are electronically generated in a 3:1 ratio with singlet states. Therefore, while fluorescent materials (singlet emitters) are theoretically limited to an internal quantum of efficiency (IQE) of about 60%, phosphorescent materials (triplet emitters) provide a theoretical IQE maximum of 100%. The incorporation of heavy transition metal ions (e.g., Ir(III), Pt(II), Os(II), Au(III), Ru(II), and Cu(I)) in organic complexes has proven to be one of the most effective methods to generate phosphorescent materials. This is due to their intrinsically strong spin-orbit coupling (SOC) that promotes intersystem crossing (ISC), which, when coupled with metal-to-ligand charge-transfer (MLCT), results in the radiative release of triplet energy. Iridium (III) complexes are the most commonly utilized organometallic phosphors in OLEDs due to their impressive photoluminescence quantum yield (PLQY), stability, short triplet state lifetimes, and spectral tunability from blue to near infrared. Additionally, to suppress inherent concentration quenching and triplet-triplet annihilation, these complexes are added as hosts and/or emitters into semiconducting host matrices, where covalent attachment to the host mitigates phase separation over time to further improve device longevity.
One popular option for display technology relies on the use of white light to render colored images through the use of color filters or color conversion techniques. In another method, white OLEDs are obtained by blending red, green, and blue emission, using a variety of device architectures and pixel layouts. For example, Samsung has used a PenTile RGBG array of red, green, and blue rectangular pixels to achieve white emission for the Nexus One smartphone display. Alternatively, pixels with a reasonable facsimile of white appearance can be obtained by blending other colors, for example, sky-blue and yellow-orange. Such pixel patterns layouts are achieved industrially using thermal evaporation of small molecules through a shadow-mask under high vacuum, which is both expensive and time-consuming on large-scale. Although researchers have developed a number of methods to achieve emissive patterned arrays from solution, including, screen printing, contact lithography, jet printing, and photocrosslinking, practical limitations such as fabrication complexity, lack of triplet state emission, scalability issues, and the use of undesirable reagents have prevented commercialization. For example, printing and contact lithography techniques require use either complex equipment or numerous iterative processing steps to achieve the desired pattern, while photocrosslinking has been used to more rapidly provide fluorescent semiconducting patterns through step-wise spin-coating/irradiation cycles. However, for photocrosslinking intense ultraviolet (UV) radiation is often used, along with the use of radical or cationic photoinitiators (e.g., cyclopentadienyl titanium or iodonium hexafluoroantimonate derivatives), that contaminate the emissive layer (EML) in an OLED device.
It is therefore desirable to have OLED displays that can be manufactured without the use of complex equipment or numerous iterative processing steps to achieve a desired pattern.